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Atomic clocks for Galileo
24 September 2004
The conference “International Workshop on Galileo Time (IWGT), Modelling and Characterization of Atomic Clocks”, in Turin on 27-28 September , will be an excellent opportunity for discussion and an exchange of views between the leading players in the ESA satellite navigation project Galileo.
The conference, hosted by the Italian employers’ federation Confindustria, will provide an opportunity for everyone involved at a European level to compare notes on the main aspects of the Galileo project. In particular, the scientific part of the conference will focus on outstanding aspects of ‘Galileo Time’ concerning modelling of atomic clocks and time signals, the heart of the Galileo system. ESA’s Director of European Union and Industrial Programmes, Giuseppe Viriglio, will take part in the concluding session of the Conference along with Rocco Buttiglione, Minister for European Policies.
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How atomic clocks work
Navigation satellites have extremely precise atomic clocks on board, which are so named because they use the oscillations of a particular atom as their “metronome”. This form of timing is the most stable and accurate reference that has ever been developed.
The operation of satellite navigations systems is based on the method of triangulation. Knowing the distance from at least three points, i.e. three satellites, the receiver on the ground can calculate its position. The distances are calculated by measuring the time that a certain signal, known to the receiver and transmitted by the satellite, takes to travel the distance between the satellite and the user. Each signal contains information on the time reference of the atomic clock on board the satellite and information on the satellite’s orbit. This allows the user to determine the position of the satellite and his own distance from it with a high degree of accuracy. The synchronisation of the atomic clocks on board the satellites is therefore critical.
The Galileo clocks
Each of the 30 Galileo satellites will have two atomic clocks on board, one based on the rubidium atomic frequency standard, the other using a passive hydrogen maser. The next-generation clocks installed on board the Galileo constellation, built by Temex Neuchatel Time (CH) and ASTRIUM-D (the Rubidium clock) and by Galileo Avionica (I) and Temex Neuchatel Time (the Passive Hydrogen Maser clock), will have an accuracy of less than a billionth of a second a day and will allow ground position to be located to an accuracy of 50 cm.
The clock that will be used as the main reference on the Galileo satellites, the Passive Maser, will offer superior performance to the current GPS clocks due to its greater stability and the fact that it will not need to be constantly synchronised and reset. The signal sent to the Earth by each Galileo satellite will include an integrity message capable of providing rapid warning if the information provided by that satellite is not reliable or if the system as a whole exhibits any kind of malfunction. This allows the user to isolate sources of error and to use solely information from the operational satellites.
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Current Status of Galileo
Galileo’s development and in-orbit validation phase is well under way and ESA has assigned the work contracts to European industry. The first civil satellite navigation system is therefore making good progress. The Galileo Programme is being implemented in phases. The first definition stage was completed in 2003, followed by a development and in-orbit validation stage that was initiated in late 2003. Next year an experimental satellite will be launched to secure the Galileo frequency filings, to characterise the orbits to be used by the in-orbit validation satellites and to test some of the critical technologies, such as the atomic clocks. Lastly, the full deployment phase will cover manufacturing and launch for the validation of the system with the launch of a mini- constellation of four satellites along with the completion of the ground segment.
Once all satellites have been fully deployed, operation will initiate for this first global civil navigation system with the complete constellation of 27 operational satellites and three in reserve, all stationed on three circular Medium Earth Orbits (MEOs) at an altitude of 23,616 km and with an inclination of 56º to the equator. To support this there will be an extensive network of ground stations and local and regional service centres.
The operations will be achieved through a public private partnership and the selection for the future Galileo operating company is ongoing under the responsibility of the Galileo Joint Undertaking equally funded by the European Commission and the European Space Agency.
Galileo will be fully compatible and interoperable with the current GPS system, thanks to an agreement between the European Union and the United States. This agreement was signed in June 2004 at the EU-US summit in Dublin. . Galileo and GPS, as well as the Russian GLONASS system, will thus become the world standard for satellite navigation. This underlines the global dimension of Galileo with agreements under discussion with other countries such as China, India, Canada, Israel, Brazil, Mexico and Ukraine. The Galileo system will soon constitute an essential infrastructure for the countries in the European Union and the world in general for accurate and secured satellite positioning to facilitate mobility on the planet.
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